Soft handoff is a process whereby, in a code division multiple access (CDMA) communication system, one mobile station (MS) may communicate with two or more sectors and/or with two or more cells substantially simultaneously. Typically the process occurs during transfer of the MS from one cell or sector to another. The TIA/EIA/IS-2001.3-B standard for the CDMA system, published by the Telecommunications Industry Association (TIA) of Arlington, Va., describes how a soft handoff may be implemented. The standard is incorporated herein by reference.
A CDMA system usually has a number of base station controllers (BSCs) communicating with a mobile switching center (MSC) via an “A” interface. Each BSC is typically connected in a point-to-point geographical configuration to a number of base transceiver stations (BTSs), the BSC and the BTSs communicating via an “Abis” interface in Abis sessions. A specific BSC controls the activities of the BTSs to which it is connected, and is used by the BTSs for activities such as call setup and teardown, as well as for administrative tasks. The BTSs in turn communicate over-the-air with MSs via a CDMA air interface. In addition, BSCs may communicate with each other via an A3/A7 interface. Each interface corresponds to a signaling protocol known in the art. For example, the A3/A7 interface is described in the above referenced TIA standard; the Abis interface is described in a TIA/EIA-828 standard, published by the TIA, which is incorporated herein by reference.
A soft handoff occurs when the MS communicates simultaneously with two or more BTSs. The BTSs may be coupled to the same BSC, and/or to different BSCs. In systems known in the art, if all the BTSs are coupled to one BSC, the interfaces involved are the CDMA air interface and the Abis interface. If the BTSs are coupled to two or more BSCs, the communication uses three interfaces: the CDMA air interface, the Abis interface, and the A3/A7 interface between the BSCs.